1. Field of the Invention
The present invention relates to circuits for switching relatively high currents and voltages and more particularly those used for switching currents in an essentially inductive load.
The invention concerns more particularly switching control circuits for chopped power supplies and circuits for controlling the horizontal or vertical scan in television receivers.
2. Description of the Prior Art
In these circuits, the switching element is a power transistor whose collector is connected to a highly inductive load comprising essentially transformer windings (for chopped power supplies) or a coil for horizontal or vertical deflection of the electronic beam (for scanning circuits).
Very often, the base of the power transistor is controlled by a circuit providing periodic control square waves of variable width.
At the high level of the square waves, a base current is applied for enabling the transistor; at the low level, this current is interrupted so as to disable the transistor.
The transistor must be able to conduct a considerable current which moreover increases linearly from a minimum value to a maximum value, at each switching period, when the load is inductive. So that the transistor may conduct the maximum current expected at the end of the conduction period (maximum deflection current for the electronic beam in the case of scanning in a television receiver), it is necessary to apply a largely sufficient current at the base of this transistor during the conduction period.
When a square wave is applied with such a largely calculated base current, the result is heavy saturation of the transistor, which adversely affects its turn-off speed at the end of the conduction period: the heavily saturated transistor will not be able to rapidly interrupt the current which flows therethrough, despite the interruption of a current applied to its base.
It is moreover now general to accelerate disabling of the transistor by applying a high negative base current pulse slightly after interruption of the square wave which controls the application of a positive base current.
Furthermore, apart from the moment when the transistor is disabled, the square wave controlling conduction of the usual devices establishes a positive base current (for maintaining conduction) which, during the whole period of desired conduction, keeps a constant value at least equal to the maximum current required at the end of the conduction period; such a high base current is not at all necessary at the beginning of conduction and it leads to a quite useless waste of power.
The present invention provides a circuit which avoids this heavy saturation and useless consumption of base current, since at all times it adjusts the base current as well as possible by giving it the required value with respect to the main current which is to flow through the power transistor at that time.